Thermoplastic resins called biodegradable polymers which have characteristics such as enzyme degradability, hydrolyzability and biocompatibility have hitherto been known, and have been given particular attention as functional polymers utilizing their excellent properties.
As one example of the biodegradable polymers, poly((R)-3-hydroxybutyric acid), a polymer accumulated in microorganisms, has been known. Further, the presence of a number of microorganisms which accumulate polymers of various (R)-3-hydroxyalkanoic acids (hereinafter abbreviated as "(R)-3-HA") in their cells has also been known (P. A. Holmes, Phys. Technol., Vol. 16, pp. 32-36, 1985; Seibunkaisei Kobunshi Zairyo (Biodegradable Polymer Materials), edited by Yoshiharu Doi, Kogyo Chosakai, pp. 26-30, Nov. 9, 1990).
Since the polymers accumulated in these microorganisms are easily degraded by microorganisms existing in soil and water, they can be widely used as clean plastics free of environmental pollution (Seibunkaisei Kobunshi Zairyo (Biodegradable Polymer Materials), edited by Yoshiharu Doi, Kogyo Chosakai, pp. 19-26, Nov. 9, 1990).
As methods for producing those polymers, first, methods utilizing the above-mentioned microorganisms have been considered. However, they have the disadvantage of requiring complex processes such as the separation of the polymers from cells, resulting in high cost of production, because of the utilization of the microorganisms or enzyme reaction. Further, most of the polymers produced contain (R)-3-HA units and 4-hydroxybutyric acid units, and polymers containing carbonate units have not been reported.
On the other hand, chemical synthesis methods are known as other methods for producing the biodegradable polymers. For example, ring-opening polymerization of optically active .beta.-butyrolactone has been reported in some documents [Polymer Letters, 9, 173 (1971); Macromolecules, 23, 3206 (1990); Macromolecules, 24, 5732 (1991)].
These methods do not require complex processes such as those in microbiological synthesis, but have many industrial problems of low molecular weight of the polymers obtained and low catalytic reactivity, which results in high cost of production.
Now, polymers composed of cyclic carbonates and lactones have already been known. For example, JP-A-3-502651 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") states clearly the production of copolymers of cyclic carbonates with lactones, particularly butyrolactone. However, in this patent specification, units derived from the cyclic carbonates are only represented by a general formula as units constituting the copolymers, and the lactones are also only represented by a general formula. Further, copolymerization of .epsilon.-caprolactone with 1,3-dioxan-2-one, a cyclic carbonate, is disclosed in JP-A-1-284262.
However, neither of these patent specifications describes the optical activity of the monomers used and copolymers formed, or teaches or suggests such optically active polymers as proposed by the present invention.
Thus, the biodegradable optically active copolymers which are ring-opening copolymers of optically active .beta.-butyrolactone with cyclic carbonates and efficient processes for producing the copolymers have hitherto been unknown.